1. Field of the Invention
The present invention relates to a monolithic semiconductor device including one or plural power output elements and circuit blocks for controlling the power output elements both formed on one chip and a method for manufacturing the same and, more particularly, to a semiconductor device whose gate oxide film is improved in reliability when an insulated gate semiconductor element is used as a power output element and a method for manufacturing the same.
2. Description of the Related Art
In a conventional monolithic semiconductor device wherein one or plural power output elements and circuit blocks for controlling these elements are formed on one chip, all the elements and circuit blocks are electrically connected to one another by metal wiring using aluminum (Al) electrodes and the like so as to serve as a single functional device and, in this state, screening is performed to determine whether the chip is defective or not.
However, in the screening of the chip, it is impossible to sufficiently determine whether the respective power output elements and circuit blocks are defective or not.
For example, as shown in FIG. 3, in a functional device comprising an output element 1 of a power metal oxide silicon FET (power MOSFET), circuit blocks 2, 3 and 4 for controlling the output element 1, and a Zener diode 5 for protecting a gate-to-source path of the output element 1, as shown in FIG. 3, a considerably high voltage has to be applied between the gate and source of the output element 1 in order to evaluate the withstand voltage of a gate oxide film of the output element 1.
However, the Zener diode 5 makes it difficult to apply such a high voltage between the gate and source of the output element 1. This drawback will be described in detail.
Normally, as shown in FIG. 4, the Zener diode 5 has a voltage of 10 V enough to turn on the output element 1 in order to protect the gate-to-source path of the output element 1 from its breakage when a surge voltage higher than a predetermined value is applied between the gate and source of the output element 1.
The gate oxide film of the power MOSFET has a time dependent dielectric breakdown (TDDB) mode in which the gate oxide film is broken in connection with time and temperature when a voltage is applied thereto for a long time, even though the voltage is lower than the breakdown voltage.
FIG. 5 is a graph showing a relationship between a temperature and an estimated lifetime of the gate oxide film for each voltage to be used.
Calculating the lifetime of the gate oxide film by means of the Anolick's model which is widely used as a formula for the TDDB, a voltage of about 20 V has to be applied to the gate oxide film at the time of probe cut in order to secure the lifetime of 15 years when the used voltage is 10 V and the temperature is 90.degree. C.
Therefore, a voltage of 20 V has to be applied to the gate of the output element 1 for chip screening. Since, however, the Zener diode 5 for protecting the gate-to-source path of the output element 1 has a characteristic as shown in FIG. 4, if the voltage of 20 V is applied, an overcurrent of nearly 20 mA flows through the Zener diode 5. The overcurrent causes the Zener diode 5 to be burnt and broken, and adversely affects the characteristic of the functional device.